Charge sensitive preamplifiers are commonly used as impedance converters for signals generated by a charge output sensor. A piezoelectric sensor is one such type of device and is widely used as a means of measuring some mechanical quantity, such as acceleration, pressure or force. As dynamic devices, piezoelectric sensors sense only changes in applied stresses, not static stresses. In addition to sensing changes in applied stress, many piezoelectric materials also sense changes in temperature i.e., pyroelectric response, and will produce a charge due to the change in temperature. In order to obtain the most accurate readings for mechanical stress, the charge due to the pyroelectric response must be attenuated since the charge resulting from the pyroelectric response and mechanical stress are indistinguishable.
In recent years data acquisition systems have had a profound increase in resolution resulting in a need for an improvement in sensor resolution. The resolution of a conventional charge amplifier, however, is inherently limited.
The circuit shown in FIG. 1 is a schematic of a classical conventional charge sensitive preamplifier (hereinafter referred to as the "Classical CSP") used to amplify high impedance signals in a large number of applications, including piezoelectric force, pressure and acceleration sensors, pyroelectric detectors, piezoelectric hydrophones and nuclear particle detectors. The use of charge sensitive preamplifiers dates back to the 1940's. The amplifier of FIG. 1 contains a physical resistor (R.sub.f) in parallel with a capacitor (C.sub.f) in a feedback loop. The output voltage is related to the desired piezoelectric signal, (q.sub.p) by the ratio v.sub.o =q.sub.p /C.sub.f. Any undesirable low frequency charge signal (q.sub.t) usually associated with pyroelectric response is attenuated via R.sub.f which forms a high pass filter with C.sub.f.
Inherently, piezoelectric sensors have very low noise. Therefore the resolution of the measurement is determined by the amplifier used to transform the high impedance charge output of the sensing element into a low impedance output voltage.
The primary source of noise that limits the performance of the Classical CSP at low frequencies is R.sub.f. Analysis shows that to have the lowest noise, R.sub.f has to have a very high resistance value, in the range of several thousands of megaohms. This approach to reducing noise becomes impractical, however, because the amplifier's leakage current will generate a DC voltage across the feedback resistor. Furthermore, increasing the feedback resistor increases the low frequency response, allowing undesirable pyroelectric signals to pass.
Several attempts have been made to eliminate the feedback resistor to improve resolution of other types of charge sensors, but each has introduced a new set of problems. As a result there remains a need in the industry for a commercially practical improved device.
One solution would appear to be to eliminate R.sub.f from the feedback loop. But this solution is also not practical because if R.sub.f is eliminated there no longer exists a discharge path for C.sub.f. R.sub.f provides a path for the DC current flow (I.sub.DC). Without the resistor the current would charge C.sub.f until the amplifier output saturated.
U.S. Pat. No. 4,801,838, Beauducel et al., proposed a configuration that uses a switched capacitor impedance transformation scheme to lower the noise gain of the amplifier. The patent claims to maintain the signal gain for the resultant increase in resolution. Practice, however, has shown that the decrease in noise gain is accompanied by a decrease in signal gain so the resolution is not effectively improved. In addition the system is much too complex to be commercially practical.
U.S. Pat. No. 4,816,713, Change, Jr. likewise fails to provide an improvement over the classical charge sensitive capacitor and teaches a sensor having a complex construction which is difficult to implement.
U.S. Pat. No. 4,053,847, Kumshara, et al., discloses a circuit for use in semiconductor radiation detectors in which feedback is used to adjust the drain voltage of the input JFET, so that the feedback resistor may be eliminated. However, the configuration of this invention is not compatible with piezoelectric sensors since it requires a low level DC current source at the input (supplied by the radiation detector) to match the gate leakage current of the JFET and also because its output has a DC bias voltage that is variable according to the voltage commanded to the drain. Further, this circuit fails to provide a means of attenuating the pyroelectric output of piezoelectric sensors at low frequencies.
A variation of the invention disclosed in U.S. Pat. No. 4,053,847 is disclosed in U.S. Pat. No. 5,347,231, Bertuccio, et al. By there invention, Bertuccio et al., sought to provide a device that would eliminate the inherent leakage current of the charge sensitive preamplifier through the use of a dual feedback scheme. Bertuccio's feedback scheme also eliminated the resistor of the conventional feedback network. However, Bertuccio, et al.'s circuit is similar to that of Kumshara, et, al. in that it fails to provide sufficient attenuation of undesired low frequency signals and has a DC output level which is non-zero and dependent upon the input transistor characteristics which can vary with processing.
Bertuccio, et al.'s circuit appears to have been designed with the needs of the nuclear industry in mind. The standards in the nuclear industry differ from those in the piezoelectric industry, as a result, Bertuccio, et al.'s invention does not provide a commercially practical solution for the piezoelectric industry to the age long problem.
Thus, the foregoing inventions have each failed to meet the needs of the piezoelectric industry for an improved charge sensitive preamplifier.